Since lead-acid batteries were invented by Plante, a French Engineer in 1859, they have been widely used as cheap and reliable secondary batteries in automobile, electric vehicle, energy storage, and other fields. As indicated in the latest statistics made by the Industrial Technology Research Institute of Taiwan, the consumption of lead-acid batteries has been taking a dominant share, though lead-acid batteries confront the competition from Li-ion batteries and nickel-hydrogen batteries in recent years. In 2012, the product value of secondary batteries in the world was USD 60.285 billion, wherein the product value of lead-acid batteries was USD 39.294 billion, accounting for 65.2% among the secondary batteries. According to the statistical data from the International Lead and Zinc batteries Study Group, in 2012, the lead consumption in the world was 10.62 million tons, about 82% of which was used for producing lead-acid batteries. According to the statistical data from China Nonferrous Metals Industry Association, in 2012, the total consumption of lead in China was 4.646 million tons, in which 3.3 million tons were used to produce lead-acid batteries. It is anticipated that waste and worn lead-acid batteries will be an important mineral asset of the society and increasingly become the principal raw material for lead smelting.
Before year 2000, lead smelting essentially employed a conventional sintering-blasting furnace process, which, in combination with fugitive emission of fume in some enterprises, resulted in severe SO2 and lead dust pollution to the environment. A process of oxidizing in bottom blowing furnace and reduction smelting in blast furnace invented by some companies such as Henan Yuguang Gold & Lead Co., Ltd. and China ENFI Engineering Corporation solves the problem of pollution of SO2 and lead dust in pyrometallurgy of lead, and has features such as short process flow and clean production. Though modern pyrometallurgy enables large-scale continuous production and is matured in technology, it involves pyrolytic smelting of lead-containing materials at 1100-1300° C., which not only brings a problem of high energy consumption, but also produces lead-containing dust in particle size equal to or smaller than PM2.5 by high temperature volatilization and lead-containing waste residue in the smelting process, consequently, the lead recovery rate is usually 95-97%.
To overcome the drawback of high energy consumption and lead emission in pyrometallurgy of lead, hydrometallurgy of lead is employed and regarded as a cleaner next-generation lead recovery process. Existing secondary lead hydrometallurgy processes, represented by hydrofluosilicic acid lead electrolysis, are unacceptable in industrial production owing to their high processing cost incurred by complex lead paste treatment process, high power consumption as high as 700-1,000 kWh/ton lead, and environmental pollution and equipment corrosion resulted from the fluorine-containing solution. Though a new process of direct PbO electrolysis in an alkaline environment reported by a research group led by Pan Junqing makes a great progress in raw material consumption, energy consumption of electrolysis, environmental pollution and other aspects, the lead recovery cost is almost close to that of the existing pyrometallurgy of lead. In years of engineering practice, it is found that the principal factor hindering the development of the new wet lead electrolysis process is still the cost, i.e., that process can't compete with an out-of-date and disordered direct pyrometallurgical recovery process without desulphurization employed in some small and medium-sized enterprises in terms of cost. In order to recycle waste and worn lead-acid batteries efficiently and thereby effectively realize regeneration and recycling of the lead resources, a breakthrough must be made out of the conventional concept of lead smelting that has dominated for thousands of years.
It can be found in the analysis of existing lead smelting enterprises that the lead provided in the existing pyrometallurgy of lead is 100% refined lead; whereas lead oxide is required as an active material in batteries in the modern lead-acid battery industry, and refined lead is required only in manufacturing plate grid and conducting tabs sectors. Therefore, while lead smelting enterprises consume a large quantity of energy and materials to smelt lead-containing materials (e.g., lead oxide) into crude lead and then electrolyze the crude lead into refined lead, the major customers—lead-acid battery manufacturers melt the refined lead into lead balls and then mill and oxidize the lead balls into lead oxide and use the lead oxide as an active material for lead-acid batteries. It can be seen that the lead smelting enterprises follow the lead smelting concept that has dominated for thousands of years but haven't taken consideration of the actual demand of their lead-acid battery customers for lead oxide. These enterprises have produced a large quantity of refined lead blindly and accordingly have consumed energy heavily and brought environmental pollution from smelting. Therefore, for the lead-acid battery industry in which the criteria for clean production and product quality become higher increasingly, the conventional lead pyrometallurgy industry must get out of the traditional concept that involves high energy consumption and severe pollution and replace the conventional lead smelting process with a direct lead oxide production process. For recycling of waste and worn lead-acid batteries, with the new concept, the pyrolytic smelting, electrolysis, and ball milling procedures, which involve high energy consumption and production of PM2.5 lead dust, lead skim, and toxic fluorides, can be omitted, and thereby the energy consumption can be reduced significantly, the recovery rate of lead can be improved greatly, and the cost of raw material for batteries can be reduced greatly. Ultimately, the waste and worn batteries recycled by battery manufacturers can be used as a raw material for producing new batteries.
The lead in lead-acid storage batteries mainly includes metallic lead in plate grids and conducting tabs and lead paste in the positive and negative poles, wherein, the recovering of the lead in the lead paste is the key in the entire recycling process. How to seek for an effective method to effectively and quickly convert the Pb (10-15 wt %), PbO (10-20 wt %), PbO2 (25-35 wt %), and PbSO4 (30-45 wt %) in the lead paste into PbO that can be used in the negative electrode or positive electrode in lead-acid batteries is a difficult task in the regenerative oxidation process of lead.
As disclosed in existing patent literature, trials have been made to prepare lead oxide from lead paste. For example, in CN201210121636.2, a raw material (e.g., sodium carbonate) and waste lead paste have a desulphurization reaction, then the desulphurized lead paste has a reaction with citric acid solution; next, through filtering, washing, and drying procedures, lead citrate is obtained; then, the lead citrate is calcined to obtain super-fine lead oxide. Though the target product in that invention is PbO, raw chemical materials such as citric acid, hydrogen peroxide, and sodium carbonate, etc. are consumed heavily. Therefore, that approach is uneconomical when viewed from the aspect of atom utilization.
In CN103374658A, super-fine lead oxide prepared from desulphurized lead paste through a three-stage process and a method for preparing the super-fine lead oxide are disclosed. The method comprises: procedure 1: acid leaching of desulphurized lead paste: the desulphurized lead paste has a reaction with an acid, while a reducing agent is added; after the reaction is completed, solid-liquid separation is carried out to obtain a lead-containing acid solution; procedure 2: preparation of lead carbonate: the lead-containing acid solution has a reaction with sodium carbonate, and then solid-liquid separation, washing, and drying are carried out to obtain lead carbonate; procedure 3: calcining: the lead carbonate is calcined to obtain super-fine lead oxide; the super-fine lead oxide can be PbO, Pb3O4, or a mixture of them. That method has the following features: nitric acid or acetic acid with hydrogen peroxide are used in procedure 1 for leaching; sodium carbonate is used in procedure 2 for desulphurization to obtain lead carbonate; lead carbonate is calcined and decomposed in procedure 3 to obtain lead oxide.
In CN102747227A, a method for preparing super-fine PbO from the active material in the poles of waste and worn lead-acid batteries is disclosed. The main principle of the method is to utilize a lead paste under the action of a reducing agent and other substances, dissolve the lead paste in nitric acid or hot hydrochloric acid solution, and then treat the lead paste with a water solution of metal hydroxide or ammonia, to obtain super-fine PbO powder for negative electrode of lead-acid battery. Likewise, a main drawback of that invention is: raw chemical materials including reducing agent, nitric acid, hydrochloric acid, and ammonia, etc. are consumed in the PbO preparation process; therefore, the PbO preparation process is uneconomical when viewed from the economic atom utilization aspect.
Similarly, in CN102820496A, a method for preparing a nanoscale lead compound from the lead paste in waste and worn lead-acid storage batteries is disclosed, comprising the following steps: (1) mixing lead paste, sodium acetate, and acetic acid with H2O2 in appropriate proportions, and controlling them to have a reaction for 6-10 h at 20-30° C. while stirring. After the reaction is completed, solid-liquid separation is carried out, and the pH of the solution is adjusted to 7.1-7.3, and then filtering is carried out to obtain lead acetate crystals; (2) calcining the lead acetate crystals for 2-3 h at 250-350° C., to obtain nanoscale PbO powder. Compared with the method disclosed in CN103374657A, in this method, citric acid is replaced with acetic acid that is cheaper. However, the problem of economic atom utilization still exists in this method.
Other relevant patent literatures include CN101514395A, and the method disclosed comprises: adding saturated oxalic acid solution into fine lead mud obtained from waste lead-acid storage batteries to have a reaction at 25-65° C., and then filtering to obtain a precipitate; treating the precipitate with excessive 30% nitric acid at 40-45° C., and then filtering to obtain a precipitate, and controlling the precipitate to have a reaction with 4 wt % ammonium carbonate solution at 25-65° C., and then filtering to obtain a precipitate; adding the precipitate into recycled HNO3 and let the precipitate to dissolve at 40-45° C. till no gas bubble is produced anymore, and then filtering to obtain a filtrate, adding 25% ammonia into the filtrate to have a reaction, filtering to obtain a precipitate and washing the precipitate to neutral state, and finally drying and calcining the precipitate to obtain lead oxide.
As described above, waste lead paste mainly contains four components: Pb, PbO, PbO2, and PbSO4. The contents (weight percentages) of Pb, PbO, PbO2, and PbSO4 vary in different waste lead pastes, owing to the criterion for battery discarding and the battery recipes of different manufacturers. Usually, the contents are: 10-15 wt % of Pb, 10-20 wt % of PbO, 25-35 wt % of PbO2, and 30-45 wt % of PbSO4. Since the lead in the negative electrode of battery tends to be oxidized into PbO in the air in the battery disposal process, the content of Pb in the negative electrode is usually lower than that in the positive electrode, resulting in relatively excessive PbO2. The existing process mainly consists of three stages: firstly, the Pb, PbO, PbO2 and PbSO4 in the lead paste are converted into soluble lead salt and PbSO4. Secondly, the soluble lead salt and PbSO4 are converted into lead citrate or PbCO3, or the like. Thirdly, the lead citrate or PbCO3 or lead acetate is calcined to obtain lead oxide.
It can be seen from the above description: for the target product, actually only the PbSO4 in the lead paste has to be desulphurized to generate PbO, while all of the other three components (Pb, PbO, and PbO2) are similar to PbO in structure, and PbO can be obtained by transferring the atom O. Unfortunately, in the existing methods, besides the lead sulfate is desulphurized by means of citric acid and then calcined for conversion, the rest three components are treated by complex acid leaching first (e.g., H2O2+acetic acid pre-reduction is carried out to generate (CH3CO2)2Pb), then treated by Na2CO3 re-precipitation to generate PbCO3, and finally PbCO3 is calcined to obtain PbO. Owing to the fact that the target product is PbO, all the raw materials added in that process, including H2O2, CH3COOH, and Na2CO3, etc., are wasted, which is uneconomical when viewed from the economic atom utilization aspect.
The research group led by Pan Junqing has made further research for improving economic atom utilization in the secondary lead conversion process, and has disclosed a novel method for utilizing the lead paste in lead-acid batteries in CN103146923A. That method comprises the following five procedures: 1. heating the lead paste in lead-acid battery and lead powder to have a solid-phase mixing reaction; 2. carrying out alkaline desulphurization in NaOH solution A; 3. leaching the desulphurized product with NaOH solution B, to obtain lead-containing alkaline solution and filter residue, and then treating by purification and cooling crystallization to obtain lead oxide; 4. utilizing NaOH solution C to carry out recrystallization to obtain PbO crystals at a higher purity; 5. after desulphurization, adding NaOH in the NaOH solution A to precipitate sodium sulfate crystals; in that approach, a NaOH desulphurization cycle is created, with sodium sulfate as a byproduct. The features of that method include: for the four components of lead paste, firstly, Pb and PbO2 are utilized to directly obtain PbO in solid state, and the excessive PbO2 in the waste lead paste is consumed by adding Pb; secondly, only the PbSO4 in the lead paste is desulphurized to generate PbO and Na2SO4; finally, NaOH solution is utilized to control the PbO to conduct recrystallization, and thereby purer PbO solid is obtained. That method utilizes an atom-economic reaction between Pb and PbO2 and purifies PbO by recrystallization in NaOH solution. The raw material NaOH, which is mainly consumed, is only used for desulphurization of the PbSO4 in the lead paste. Thus, unlike other processes in which all components in the lead paste are converted into lead salt and then desulphurized, the process disclosed in that patent document exploits a novel lead oxide recovery technique from the aspect of economic atom utilization. Through more than one year of research made by the research group, it is found that the method still has many drawbacks that must be eliminated by further innovation, including:
1. Long process flow: five procedures are required in that process, in which three NaOH solutions have to be used for cyclic processing, wherein, the NaOH solution A is used for desulphurization, the NaOH solution B is used for leaching, and the NaOH solution C is used for recrystallization and NaOH is added for precipitating sodium sulfate. Therefore, it is very necessary to simplify the process and thereby reduce the recovery cost and energy consumption.2. PbSO4 doesn't participate in the reaction before/after heating, in the high-temperature solid-phase conversion of the lead paste in the first stage. The PbSO4, which accounts for 30-45 wt % of the total weight of the lead paste, is mingled with Pb and PbO2 and is heated up meaninglessly, resulting in energy waste; in addition, a great deal of lead sulfate included in the lead paste results in incomplete solid-phase contact reaction between Pb and PbO2, and consequently a considerable amount of Pb or PbO2 particles remain in the product. Hence, it is of particular importance to eliminate the adverse effect of PbSO4 or convert PbSO4 into a precursor of PbO before the heat treatment.3. As for the existing process, a PbO product can be obtained through four procedures, i.e., calcining—desulphurization—leaching—crystallization. Such a process is very long. More severely, some useful additives in the waste lead paste, such as super-fine barium sulfate, are abandoned as impurities in the process. It is well known that super-fine barium sulfate is added as a swelling agent in the lead paste of negative electrode in the existing production of lead-acid batteries, in order to improve the service life of the negative electrode plate of lead-acid battery. The residual barium sulfate in the waste lead paste should be utilized appropriately to directly produce PbO containing a specific amount of barium sulfate as negative electrode composite material required for production of negative electrodes of lead-acid batteries, thereby utilizing the two components (lead oxide and barium sulfate) in the waste lead paste in an integrated manner.
In summary, it is an urgent task to invent an innovative short process to quickly obtain a PbO complex and keep the barium sulfate component in the lead paste as a useful additive for lead oxide, and, on that basis, add barium sulfate in appropriate amount to meet the demand for production of an active material for negative electrodes of lead-acid batteries. In such a way, not only the lead component in the lead paste can be utilized, but also the barium sulfate additive can be recovered, and thereby the overall recycling value of lead paste can be greatly improved.